Wood has served man for untold centuries. This unique material has been used to provide such basic necessities as shelter, warmth, weapons and tools. One of the important characteristics of wood is the ease with which it can be worked with tools. In the early 19th century, circular saw, band saw and surface planing machines were invented for wood processing. Since then, wood processing has been subjected to improvements depending on the market demand which is mostly affected by the quality and production costs.
Wood processing principally involves the separation of chips from a workpiece up to a desired dimension and surface quality. The process produces characteristic wood failures. During wood processing, the superficial layer is subjected to various types of stress, such as cutting force and friction. If these stresses exceed the elastic deformation range, permanent deformation will take place. It is considered that this permanent deformation plays an important role on the final surface quality and/or subsequent operation. In the past, many researches have chosen the shear strength at the glue line of a laminated board as a surface quality criterion. Through these studies, the board was planed under various machine conditions. It was then laminated and subjected to shear test. Despite an apparently smooth surface after conventional surface planing, previous studies show that in most cases the gluing performance was altered. This alteration has been attributed to the superficial damages due to the stresses which has been developed through machining processing. River and Miniutti in Wood and Wood Products, Vol. 80, No. 2, 1975, pp. 35-36 and 38, evaluated the lamination performance in yellow polar, and red oak specimens planed with saw, knife planer and jointer. The overall gluing performance decreased progressively from planed surface with jointer to planer and then to sawn surface.
Jokerst and Stewart, in Wood and Fiber, Vol. 8, No. 2, 1976, pp. 107-113, reported lower shear strength at the glue line of laminated boards on the surface planed with an abrasive planer as compared to a knife planer. The reduction of the shear strength at the glue line has been attributed to the normal force component which developed through cutting action. The magnitude of this normal force is related to the knife geometry, as well as the combination of the material, and machine conditions. If this vertical force exceeded the stress at the proportional limit of the wooden workpiece, the superficial cells will be subjected to a permanent crushing and breakage. As a result, any glue application for laminating or finishing products when applied on these damaged cells will result in lower strength and quality. The severity of damage depends on the specific cutting conditions. The development of this perpendicular component mostly has been attributed to large rake angle in abrasive planing, negative clearance angle in knife planer, as well as the development of the wear at cutting edge.
A fixed-knife pressure-bar system has been proposed as an alternative planing method to reduce or eliminate subsurface damage induced in wood. The fixed-knife pressure-bar surface planing improved the shear strength at the lamination line compared to a knife-planer method after a cycle of sorption. Hernandez, in Forest Product Journal, Vol. 44, No. 7/8, 1994 pp. 63-66, confirms the superior gluing performance of the maple board planed with fixed-knife pressure-bar compared to knife planing method. The difference mostly related to a negative clearance angle through the knife-planer action compared to positive clearance angle and lower normal force components through the fixed-knife pressure-bar planing method. However, this should take into account the dealing with a knife during surface preparation, which means continuous changing of the sharpness and knife geometry due to the knife wear.
In recent years, laser light was introduced to the wood industry with a view to improving the above mentioned mechanical difficulty. A laser beam acts as a fresh knife with the ability to apply the desired cutting condition with minimum kerf width. The application of laser light to wood processing was promising since each laser beam acts as an individual sharp, disposal cutting edge with high precision and performance. However, laser planed surfaces always left behind a layer of char because of the specific strong heating action of laser light. Such heating action blows away surface material while also heating the surface. The heated surface in turn oxidizes since it reacts mostly with oxygen molecules in the air, leaving behind a burnt mark. McMillin and Huber, in Forest Products Journal, Vol. 35, No. 1, 1985, pp. 23-25, evaluated the glue bond strength of a laser planed surface. The results show that the strength of laser cut oak glued with polyvinyl acetate adhesive was reduced to 75% of sawn lamination. It was concluded that a light sanding or planing was necessary to remove the burned superficial layer. The blackened-up burned superficial layer contained a powdery char. This of course was not appreciated in the wood industry.